JPH10251099A - Production of lithium tetraborate single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the evaporation of a lithium tetraborate single crystal raw material and to suppress the occurrence of the cracking of the single crystal by coating the surface in the upper part of the lithium tetraborate single crystal raw material in a crucible of an apparatus of a perpendicular Bridgman method or perpendicular temp. gradient solidification method with a carbon material and growing the single crystal. SOLUTION: The lithium tetraborate single crystal raw material (a) compounded at prescribed molar ratios is put into the cylindrical body part 5a of the platinum crucible 5 arranged on a crucible base 4 which is installed in an electric furnace 1 and is movable vertically with the apparatus for growing the single crystal by the perpendicular Bridgman method, etc., and a seed crystal (b) is inserted into a seed pipe 5b. Next, the surface in the upper part of the lithium tetraborate single crystal material (a) in the crucible 5 is covered with the carbon material C, such as graphite, A disk-shaped molded part which fits into the cylindrical body 5a and is removably housed therein is used as the carbon material C. The raw material (a) is then melted and a temp. gradient is set at about 20 deg.C at the growth point. The crucible 5 is moved at a crucible lowering rate of about 0.3mm/hour to allow the single crystal to grow and thereafter, the single crystal is cooled down to room temp. The lithium tetraborate single crystal is taken out.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a single crystal of lithium tetraborate, and more particularly to a method for producing lithium tetraborate useful as a substrate material for a surface acoustic wave device.

[0002]

2. Description of the Related Art Conventionally, as substrate materials for surface acoustic wave devices, lithium niobate, lithium tantalate, quartz,
Lithium tetraborate and the like have been put to practical use. Among these single crystals, lithium tetraborate is useful as a filter for mobile communication devices because it has a relatively large electromechanical coupling coefficient and a very small delay time temperature coefficient at room temperature. Material.

As a method of growing a lithium tetraborate single crystal, a method of growing a lithium tetraborate single crystal from a melt by the vertical Bridgman method or the Czochralski method has been conventionally known.

In the Czochralski method, a raw material is put into a crucible, heated and melted, a seed crystal is brought into contact with the melt, and the melt is gradually pulled up while rotating to grow a crystal. Despite the advantage that the growth rate can be increased compared to the method, thermal distortion occurs in the crystal due to a sharp temperature gradient, and cracks occur during growth due to temperature fluctuation due to gas convection in the furnace And the yield of crystals was low.

On the other hand, the vertical Bridgman method moves a crucible containing a melt in a furnace having a temperature gradient and solidifies the lithium tetraborate melt from a crucible tip (lower end) into which a seed crystal is inserted. In addition, since the temperature gradient is relatively gentle and the temperature fluctuation is small, a high-quality single crystal can be obtained as compared with the Czochralski method.

[0006]

However, in the case of the conventional vertical Bridgman method, a part of the raw material evaporates during the growth and causes a composition shift, and finally a part to be crystallized, that is, a melt in the crucible is formed. The upper part is vitrified. Therefore, during cooling, cracks are generated due to thermal stress due to a difference in thermal expansion coefficient between the vitrified upper portion and the crystallized portion below the upper portion, and there is a problem that the upper portion of the crystal cannot be provided as a product.

[0007] The present invention has been made in view of such circumstances, and an object thereof is to prevent evaporation of raw materials put in a crucible, prevent vitrification, and stably produce high-quality crystals. An object of the present invention is to provide a method for producing a lithium tetraborate single crystal that can be grown.

[0008]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted intensive studies and as a result, have found that the specific gravity is smaller than that of a single crystal of lithium tetraborate and that the single crystal is fused to the single crystal. By covering the upper surface of the melt with a difficult carbon material, it is possible to prevent evaporation of the raw material and prevent vitrification, and therefore, it is possible to stably grow high-quality crystals without cracks. It was completed.

That is, the present invention provides a method for growing a single crystal of lithium tetraborate from a raw material in a crucible by a vertical Bridgman method or a vertical temperature gradient solidification method according to the present invention. Is characterized in that its upper surface is covered with a carbon material.

Examples of the carbon material include those commonly used in this technical field, for example, graphite, carbon and the like. In addition, as a usage form of the carbon material, if the upper surface of the raw material in the crucible can be covered in order to prevent a part of the raw material from evaporating during the growth, a powder form,
It may be in either bulk or fibrous form.

According to the method of the present invention, by covering the upper surface of the raw material in the crucible with the carbon material, a part of the raw material can be prevented from evaporating during the growth, and the carbon material is Since the specific gravity is smaller than that of the melt of lithium tetraborate single crystal and it is hard to react with the melt, it is difficult to fuse and there is no risk of contaminating the raw material or being contained in the raw material, so that there is no crack. High quality lithium tetraborate can be stably obtained.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for producing a lithium tetraborate single crystal according to the present invention will be described below with reference to the drawings, taking a vertical Bridgman method as an example.

FIG. 1 shows an example of an apparatus for carrying out the manufacturing method of the present invention. In the figure, reference numeral 1 denotes an electric furnace in which a heater 2 is provided. The crucible table 4 supported by is accommodated.

The crucible table 4 is made of a heat-resistant material usually used in this kind of technical field, and is fixed to the upper surface of the support table 3. A concave portion 4a in which a crucible 5 to be described later is fitted and removably accommodated is formed in the upper surface portion of the crucible base 4.

The crucible 5 includes a large-diameter body portion 5a and a crucible 5 similar to those conventionally used in this type of manufacturing method.
A small-diameter seed tube 5b provided below the body portion 5a, and is accommodated in the recess 4a.
It is supported to stand on the crucible stand 4. The upper opening of the crucible 5 has a lid 6 made of the same material as the crucible base 4.
It is closed by.

A vertical movement mechanism 7 is provided at a lower end of the support base 3, and the operation of the vertical movement mechanism 7 causes the crucible base 4 and the crucible 5 to move up and down integrally with the support base 3. The upper opening of the accommodation space 1 a of the electric furnace 1 in which the support table 3, the crucible table 4 and the crucible 5 are accommodated is closed by a predetermined lid 8.

Hereinafter, the production of a single crystal of lithium tetraborate by the production method of the present invention using the above apparatus will be described with reference to more specific examples.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, purity 4N (99.99 wt.) Prepared at a predetermined molar ratio in the air or in an inert atmosphere.
%) Of the lithium tetraborate single crystal raw material a
m, placed in the body 5a of a platinum crucible 5 manufactured to a length of 300 mm, and having an orientation of <5 mm in diameter and 100 mm in length <11
The seed crystal b of lithium tetraborate processed to the 0> orientation is inserted into the seed tube 5b.

Finally, the carbon material c is put in the crucible 5,
The upper surface of the lithium tetraborate single crystal raw material a in the crucible 5 was covered with the carbon material c. Here, as the carbon material c, graphite molded into a disk-like shape that is fitted in the body portion 5a and removably accommodated therein was used. Further, this raw material a was melted at 920 ° C. or higher so as not to melt the seed crystal b.

Next, at the growth point, a temperature gradient of 20 ° C.
With the crucible descending speed set to 0.3 mm / hour, the platinum crucible 5 was moved by 200 mm to grow a single crystal, and then cooled to room temperature.

Further, the platinum crucible 5 was broken and lithium tetraborate was taken out to obtain a working product manufactured by the method of the present invention.

A comparative product was obtained by growing a single crystal in the same manner as in the above embodiment, except that the carbon material c did not cover the upper surface of the lithium tetraborate single crystal raw material a in the crucible 5. Was.

Each of the product of the present invention manufactured in this way and the above-mentioned comparative product manufactured by the conventional manufacturing method was examined by the naked eye for the presence or absence of cracks. It was not produced and was high quality lithium tetraborate. On the other hand, it was confirmed that the comparative product had a crack in the upper portion, and that portion was unusable as a product.

[0024]

As described above, the present invention provides a high quality lithium tetraborate having no cracks by covering the upper surface of the raw material in the crucible with a carbon material to prevent the raw material from evaporating and preventing vitrification. It can be obtained stably. Therefore,
Since all of the raw materials can be provided as products, they can be suitably used in this kind of field as a method for producing a lithium tetraborate single crystal having a high yield.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional front view showing an example of an apparatus for carrying out a method for producing a lithium tetraborate single crystal according to the present invention.

[Explanation of symbols]

 1: electric furnace 2: heater 3: support base 4: crucible base 5: platinum crucible 5a: body 5b: seed tube a: lithium tetraborate single crystal raw material b: seed crystal c: carbon material

Claims (1)

[Claims] 1. A method for growing a lithium tetraborate single crystal from a raw material in a crucible by a vertical Bridgman method or a vertical temperature gradient solidification method, wherein the upper surface of the raw material in the crucible is covered with a carbon material. A method for producing a lithium tetraborate single crystal.